The present invention relates to a meter for continuous measuring of the mixture proportions within fluids, e.g. measuring of the contents of water in oil or similar liquids flowing through a tube. The meter or sensor is simple and accordingly non-expensive to manufacture and well suited for measurements with simple electronics. The invention also relates to a method for undertaking such measurements.
Several meters for measuring of the water contents in oil are available on the market today. Some of these meters are based on the use of radioactive radiation, some of them are capasitive and some are based on use of microwaves.
Radioactive sensors are not acceptable or rather controversial in many environments due to the health hazard represented by the radiation, and the required security precautions. An sufficient accuracy also represents a problem as the radiation is most sensible for variations in density, and the difference in density between water and oil is rather low.
The capasitive meters detect the permittivity of the mixture of fluids at a frequency much lower than the frequency regularly used in sensors based upon microwaves. To explain the subject permittivity, it may e.g. be referred to ref. 1: Nyfors, E., P. Vainikainen, Industrian Microwave Sensors, Artech House 1989, chapt. 1. Such sensors are rather sensitive for different coatings as a large increase of the impedance will be the result of even a thin coating. The capasitive sensors also require a relatively complex design using a dielectric internal protection in the sensor to avoid direct contact between the electrodes and the fluid or the liquid which is to be measured.
The above problems are not involved with the microwave sensors.
Microwave sensors for measuring the contents of water in oil are conventionally based on a microwave resonator used as sensor. (Ref. chapt. 3 i the above book; ref. 1). To ensure that the sensor makes measurements of the complete flow, the resonator has to be a cavity resonator implemented in the tube. To ensure a sufficient quality factor (Q-factor) for the sensor, such a resonator must have a structure which prevents a leak of microwaves from the sensor and further out in the tube when the fluid flows. One possibility is to delimit a section of the tube physically by a net-like structure or screen in each end, with so narrow openings that the microwaves cannot radiate into the tube while the fluid may pass rather undisturbed. Each such end section then represents a short circuit for the microwaves. However, such a structure is very intrusive and represents a bar for cleaning and also for the flow if the flow includes solid particles. Such a cavity resonator has many different tuning modes with corresponding resonance frequencies. Accordingly a simple, self-oscillating electronic circuit may possibly not be used to determine the resonant frequency, as the risk to find an erroneous resonant frequency is large, in particular if the resonant frequency may vary within a broad range of frequencies. Accordingly a more complex measurement method has to be used, e.g. based on a VCO-circuit (=Voltage Controlled Oscillator) controlled by a personal computer (PC) in such a manner that the measuring signal is scanned over a certain range of frequencies.
An object of the present invention is to provide a meter based on a new microwave resonator having a simple structure, being less intrusive and less expensive than cavity resonators defined by net-like end sections. The new resonator is also well suited for measurements by a simple, self-oscillating electronic circuit, as it is possible to eliminate the two resonance frequencies closest to the desired frequency, so that the risk that the measurements are undertaken on an erroneous frequency are reduced or eliminated. Accordingly the invention has all the advantages of a microwave sensor, but is less intrusive than an conventional cavity resonator, is less expensive in production, and may use a simpler and less expensive electronic circuit.
The invention is in particular developed to measure the contents of water in oil, and then again especially for applications where the price is a delimiting factor and the requirements to the accuracy are not very high. Such an application may e.g. be measuring of the water contents in the fuel of a ship engine. According to one technique a small portion of water is added to the fuel in a ship""s diesel engine to reduce the pollutant discharges of NO,. One portion of the fuel will always be recirculated in a diesel engine and accordingly the fuel will already have a water contents when mixed with new fuel entering the system. To control the injection water the water contents accordingly must be measured. The requirements to the precision of these measurements are about xc2x12%, and the cost is very important. This invention is accordingly very well suited for such applications.
More generally the present invention may be used to measure the water contents in oil for all applications, and also the mutual proportions between two other fluids or liquids when only the continuous phase is non-conductive and the two fluids or liquids have different permittivities.
All of the above mentioned objects and advantages are met by using a method or a sensor according to the patent claims stated below.
In the following the present invention will be described in more detail by a thorough description of an embodiment which is an example. Here measuring of the water contents in oil is again used as an example. Similarly a cylindric sensor is used, also as an example. Accordingly this example does not exclude sensors having other cross sections than circular-sylindric. The sensor may e.g. have a polygonal or oval cross section without leaving the scope of the invention.